1. Field of the Invention
The present invention relates to an electron linear accelerator, and more particularly to an electron linear accelerator having enhanced electron beam focusing properties.
2. Description of the Related Art
An electron linear accelerator has been disclosed in U.S. Pat. No. 4,641,103 and the like. FIG. 11 shows an example of the electron linear accelerator according to the prior art.
In FIG. 11, a cathode for emitting thermions is provided on a cathode support cylinder 15. The thermions are led out to an RF electron gun cavity 12 in a direction of an electron accelerating tube 14 by a microwave generated from an RF source 13.
FIG. 12 is an enlarged view showing the vicinity of a tip of the cathode support cylinder 15 shown in FIG. 11. In FIG. 12, the tip of the cathode support cylinder 15 is provided with a cathode 10 for emitting thermions. The cathode 10 is heated by a cathode heater 16. The thermions are emitted from a face 11 of the heated cathode 10. The RF electron gun cavity 12 is provided ahead of the thermion emitting face 11 of the cathode 10. An electric field 30 of the microwave generated from the RF source 13 for leading a thermion 25 in the direction of the accelerating tube 14 exists in the RF electron gun cavity 12. The thermion 25 is led out of the vicinity of the thermion emitting face 11 of the cathode 10 by the electric field 30, and is incident on the accelerating tube 14 through an RF electron gun and is accelerated to a high energy.
In such an electron linear accelerator according to the prior art, however, distribution of the electric field 30 for leading out the thermion 25 in the vicinity of the cathode 10 easily becomes uneven as shown in FIG. 12. In other words, the distribution of the electric field 30 is not parallel with the thermion emitting face 11. For this reason, the thermions 25 are not moved in parallel with each other toward the accelerating tube 14 (FIG. 11) so that an electron beam diverges. Therefore, beam performance such as an electron emittance or focusing properties of the electron beam are deteriorated.